1. Field of the Invention
The present invention relates to a sand making apparatus that makes sand having a uniform particle size by sorting crushed minerals into plural kinds according to particle sizes, a sand making method, and made sand. More particularly, the present invention relates to a sand making apparatus, a sand making method, and made sand that enable effective use of excess by-product powders generated in a crushing process.
2. Description of the Related Art
The present applicants previously applied for a patent on a device and a method for sorting minerals (hereinafter, referred to as “crushed materials”) crushed by a crusher into particles (hereinafter, referred to as “coarse particles”) having a particle size larger than a desired particle size, particles (hereinafter, referred to as “fine particles”) having a particle size equal to the desired particle size, and particles (hereinafter, referred to as “powders”) having a particle size smaller than the desired particle size, by a combination of air sorting and screening sorting.
A sorter that is disclosed in Patent Document 1 crushes crushed materials, appropriately sorts the crushed materials according to sorted particle sizes, and causes powders generated in a crushing process to be sucked and collected by the negative pressure from a dust collector and stored as excess by-product powders (fillers) in a reservoir (Japanese Patent Application Laid-Open No. 2003-10725).
In the related art, if a large amount of powders that have an average particle size of about 75 μm or less are mixed in made sand, this adversely affects properties (flow value or slump value) of concrete using the made sand as a fine aggregate. For this reason, when a mineral ore is crushed and sand is made, as described in Patent Document 1, a method that sorts crushed materials into coarse particles, fine particles, and powders and collects only the fine particles as made sand is adopted.
However, according to a discovery from recent studies, in regards to a relationship between the fine aggregate and the properties of the concrete, even though only powders having relatively large particle sizes (fine particles that have an average particle size that is not smaller than about 20 to 40 μm and not more than 300 μm) among excess by-product powders generated in the sand making process are mixed with the fine aggregate, this rarely affects the properties of the concrete.
From the discovery and a viewpoint of effective use of the excess by-product powders, it is desirable that the sorter sort crushed materials into coarse particles, fine particles, powders (hereinafter, referred to as “coarse powders”) having a relatively large particle size, and powders (hereinafter, referred to as “fine powders) having a relatively small particle size, and finally collect the fine particles including the coarse powders as the made sand.
However, if one sorter is used to perform work for sorting the crushed materials such as the mineral ore into four kinds of materials (sorting the crushed materials into the coarse particles, the fine particles, the coarse powders, and the fine powders), the size of the sorter may greatly increase as compared with a sorter that performs work for sorting the crushed materials into three kinds of materials (sorting the crushed materials into the coarse particles, the fine particles, and the powders), and it is difficult to actually operate the sorter.
Accordingly, it is an object of the present invention to provide a sand making apparatus that can easily sort sorted powders into coarse powders and fine powders and mix the coarse powders as made sand, while using the conventional sorter that performs the three kinds of sorting (coarse particles, fine particles, and powders), a sand making method, and made sand.